This invention has relation to exhaust fan assemblies wherein shutters are provided which are forced open by the action of air passing through the assembly when the fan is operating and which are biased to return to a closed position when it is not.
Such assemblies are not new, but anti-backdraft exhaust fan assemblies of the prior art suffered from certain difficulties which have been overcome in the present invention.
A most desirable attribute of such assemblies is that the air pressure needed to hold the shutters in open position be kept at a bare minimum to maximize the energy efficiency of the unit. In other words, the energy of motion of the air against the shutter blades to hold them open is entirely lost, so the less flat plate area presented by the shutter mountings and shutters while in their open position, the better.
Also in this regard use of two shutters is much better than use of many shutters in providing for minimum restriction to air flow.
Third, where only two blades are used, these shutters can be positioned at the exhaust end of the assembly, or at the intake end. If positioned at the intake end, the motor and fan assembly is substantially elongated to have room to let the blades swing clear back to substantially parallel position with respect to the air flow and still clear the electric motor, in accordance with the teachings of the prior art. If positioned at the exhaust end, they will be out in the weather. Such a structure is unsatisfactory and/or unworkable in areas where there is considerable snow and sleet during a given year.
Fourth, when a series of horizontally pivoted narrow shutters are positioned at either the intake or exhaust end, virtually the entire weight of the shutter must be supported by the air flow, and this results in a waste of energy.
Additionally, shutters, whether horizontally or vertically pivoted, do collect dust over long periods of time. The added weight of such dust does not appreciably affect vertically pivoted shutters, but greatly inhibits full opening of horizontally pivoted shutters.
Shutters, whether horizontally or vertically mounted, on the outside or exhaust port end of an exhaust fan assembly are subject to action by transient cross winds and by winds directly against them which can sporatically cause the shutters to open to allow undesirable backdraft or can present a pressure on the shutters which will tend to hold them closed against the action of the exhaust fan when it is turned on.
Therefore, what has been needed, and what is provided by the present invention, is an exhaust fan and anti-backdraft shutter assembly in which the shutters are pivotally mounted in adjacent relation to the intake port of the assembly and between the motor-fan and this intake, and which does necessitate use of a bulky structure on which the shutters pivot. The shutters should be openable easily when the fan operates and, when the fan is not operating, should be rather gently biased to move to a closed, anti-backdraft position.
No preliminary search was made on this invention. However, the inventor and those in privity with him are aware of no prior art which is closer than that dicussed above, and are aware of no prior art which anticipates the claims made herein.